Chipotle saw a key sales figure rise more than expected as it works to regain customers' trust after a series of food scares, but likely lost at least a little ground again in disclosing a possible data security issue.

The company said it is believes it has stopped the unauthorized activity on the network that supports its payment system in restaurants. It said the investigation is focused on restaurant transactions between March 24 and April 18 of this year. It said it would not provide further details since the investigation is still going on.

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A Chipotle representative said the company has notified card networks, which notifies issuing banks, which in turn notifies customers. Chipotle shares, which had been up more than 5 percent in after-market trading, gave back some of that gain.

The Denver-based chain said earlier that sales rose 17.8 percent at established locations for the three months ended March 31, following a 29.7 percent decline in the year-ago quarter. Sales at stores open at least 13 months had turned positive in December after dropping steeply starting in late 2015. The 13-month metric is a considered a key indicator because it strips out the volatility of newly opened and closed locations.

Since the E. coli and other food scares that started sickening people around country more than a year ago, Chipotle has pledged to ensure that its food is safe, offered coupons for free burritos, stepped up advertising and introduced a temporary loyalty program to get people back in its stores. The company also added chorizo as a topping to lure more customers.

Chipotle said Tuesday that it will start testing a dessert next month. CEO Steve Ells described the dessert, bunuelos, as fried tortilla strips with honey and sugar with an apple and caramel dipping sauce.

For the quarter, Chipotle Mexican Grill Inc. earned $46.1 million, or $1.60 per share, following a loss in the year-ago period. Analysts had expected $1.29 per share, according to FactSet. Total revenue was $1.07 billion, topping the $1.05 billion analysts expected.

The company has continued building new locations, and ended the quarter with 2,291 stores.